Dolly wheels are commonly used on vehicles or other devices which operate at low speeds (0–10 m.p.h.). However, when the speed of the vehicle increases, such as to 15 m.p.h. or greater, current dolly wheels can become unstable and oscillate or shimmy. The oscillation or shimmying is a phenomenon characterized by the vibration or rotation of the dolly wheel about its axis. The oscillation or shimmying is caused in part by offset load impacts on the wheels, such as can occur when the side of the dolly wheel surface that engages the road contacts resistance, stones or bumps, which cause the wheel to rotate and then recenter when the trailing dolly effect again becomes the controlling directional force on the wheel. In rapidly recentering, the wheel typically goes past center and then must recenter back again. The rapid effect that this phenomenon sets up is an ongoing shimmy or oscillation which is disruptive to the smooth operation of the vehicle.
As the speed of the vehicle increases, the need for rapid response of the dolly wheels and full rotation of the dolly wheels is lessened. At higher speeds, as discussed, however, road impacts to the dolly wheels can excite oscillation and instability of the dolly wheel axles. It would therefore be desirable to provide a dolly wheel system which resists the dynamic excitation caused by use of the dolly wheels at high speed.
As is known, with dolly wheel steering systems the front wheels follow the vehicle's motion direction as the vehicle rotation is normally provided by the drive axle. Dolly wheel in concert with drive axle steering utilizes free rotating dolly wheels that are configured to swivel 360 degrees with the trailing wheel center allowing for automatic alignment. In addition to instability at high speeds, this zero turn capability about the driven axis can cause instability when the free rotating dolly wheels encounter rough terrain which can result on loss of directional control. For example, because the dolly wheels respond to the unequal speeds of the drive wheels, undesired vehicle rotation and hence turning can result, such as when one drive wheel experiences slippage due to lack of traction or when the vehicle is driving along a side hill where the tires see unequal weight and thus unequal traction, or a side hill where the vehicle weight over the dolly wheel wants to pull that end downhill, whereas a steered tire would hold the desired vehicle path. At slower speeds and in normal conditions, however, the dolly wheels are required to have complete freedom to follow the drive wheel turn requirements.
It would therefore be desirable to have a dampening and/or locking system that can operate upon receipt of signals indicating the presence of conditions that are detrimental to dolly wheel stability or to undesired vehicle turning.
Presently, various methods and designs have been developed to reduce these problem of shimmying and oscillating. Some of these designs have used hydraulic braking devices and friction devices in order to restrain movement of the dolly wheels regardless of the speed of the vehicle. Moreover, some of these hydraulic dampers operate only at certain positions of the dolly wheels and others operate for the entire 360 degrees of rotation of the dolly wheel axle. They are thus constrained by when they can be deployed.
In one particular example, disclosed in U.S. Pat. No. 4,667,365, a system for providing a braking force for a dolly wheel is provided. The disclosed system provides a dolly wheel assembly including a support for attaching the dolly wheel to a vehicle, a dolly wheel mounting member rotatably mounted on the support, a rotating brake member mounted so as to rotate with the dolly wheel axle, and a stationary brake member fixed relative to the vehicle. The dolly wheel axle is defined by the dolly wheel kingpin and is free to rotate throughout the 360 degree path of travel of the dolly wheel. The assembly also includes a controller for operating the brake so as permit braking of the dolly wheel axle at any position.
In this case of U.S. Pat. No. 4,667,365, the programmable controller is intended to sense the speed of the vehicle. When the programmable controller senses that the speed of the vehicle is below a preset level, it does not apply any braking force to the axle and allows it to freely rotate. When the programmable controller senses that the speed of the vehicle exceeds a preset speed, the controller applies a predetermined force to the brake, which is sufficient to prevent oscillation or shimmying. While this system and other similar systems have been developed, they all present problems in controlling the actual dampening force as well as its repeatability in varying environmental conditions. Moreover, their lack of sensitivity causes them to be limited as to when they can be employed or used and thus they are not readily applicable to all vehicle designs.